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Mind you, it's the "...and have it all work automatically" part that's the really expensive requirement. Once you lose that, and accept increased operator responsibility, it comes down to nuts and bolts of a dual supply. There are two different concerns that would alter the dual-supply concept. You mentioned the first, not enough flow from the ground water. The second is a town that will not permit you to interconnect ground water and city water. (Their water, and their call)

Without a directly piped interconnection, things get more complicated, but in a way you might not mind. You add a holding tank, maybe in the form of an outdoor cistern. A sump pump can run to fill the holding tank. A second pump atop the holding tank provides the actual pressurized water that feeds the system. This arrangement lets the system run for as little time as needed for coverage, even if the actual filling of the holding tank takes many hours more. What you spend with a holding tank, you make up in part by not needing an RPZ.

Ok. I'll figure a way to measure the flow and get the back flow preventer spec and inspection requirements from the city. I knew it wouldn't be cheap, but I think I could make the money back in a couple of years if it works. I also like the option of not irrigating with potable water if I have a reasonable alternative. I will also test how consistent the source is by seeing if I can pump it dry and measure how long it takes to recover.

Thanks for your feedback. It will take me a while to get back with the info you need.

All of what you describe will cost thousands (that's plural), so break it down and simplify. First off, the RPZ is the only device capable of isolating city water from pumped ground water. It may be necessary to have the device regularly tested and certified by a licensed inspector of backflow prevention.

Put down into numbers what the existing system requires in way of water flow and pressure. Get a pressure gauge. You can determine flow with the water meter and a stopwatch. You need these numbers to match up a pump to the system. Also, you can experiment with slightly closing the supply valve to the system, and taking another set of measurements. The idea here is to determine practical minimums for flow and pressure for each zone. (since some city water supplies are far higher in pressure than the sprinklers actually require)

Specific design and products needed for multiple water source hookup

Hi,

I have a RainBird ESP-Modular 12-zone system installed by a contractor. It is connected to the city water supply.

Also, within 15 feet of the connection between the system and our city water, we have a 5-foot deep concrete lined sump pit in our basement floor. When our house was built, they needed to put 3 feet of gravel under the slab because we are sitting on awful marine clay soil. When we first moved in, we installed a sump pump which pumped continuously, ultimately flooding our down-hill neighbors' yards. So, I just unplugged the pump to see what would happen. Turns out, the pit was continuously filled with clear water, but it never reached closer than 1 foot from the top of the pit. Even during hurricane Sandy it did not overflow. There must be some kind of underground stream flowing though it.

Well, you can see where I'm going. Why pay the city hundreds of dollars for water when my house is virtually floating already, and I have an easy top into that source.

So, I get the basic idea with a pump, backflow preventer and valve to switch between city and sump pit water, but I'm struggling with specifics:

For example:
- Since I didn't install the system, how do I know the flow requirements.
- The city water line feeding the system is 3/4" copper, does that help?
- I know I need a backflow preventer, and I assume RPZ. Beyond that, does it matter which I choose?
- I still need to coordinate with the city code people, but you guys probably already know the best way to go.
- How do I decide on a pump? The city water feeds the current system about 8 feet above the bottom of the pit.
- I would like it to automatically switch to city water if the sump pit gets too low. How best to do that?
- What other features do I need: inlet type, filters/strainers, gauges, pressure tanks, etc.
- If you can recommend specific products, I'll just buy what you say.
- I want good, quality stuff both for safety and reliability. I'm not looking for the cheapest solution possible.

I would like to do this myself. I will be fun. I have no concerns about the technicalities of the plumbing and electrical work. I just need the right design.